here is the straight copy and paste out of the html manual of the game
Covert Operations Level 4: Assassin
Your expertise with the knife allows you an instant kill with any backstab.
But doesn’t anyone get free instant kill backstabs? I was racking up knife kills today (like at least 8 knife kills per round) by disguising myself as an allied (i was on axis team) and I know I wasn’t at level 4 when I did some of my knifing runs. Does level 4 increase “backstab region” or something?
Heh, I managed to knife 9 ppl in a row today on Fuel Dump, while the tank was strolling through the tunnels. it was frekaing hilarious. I was disguised and got 3 kills, on my 4th kill, I lost my disguise, but I continued running around with my knife and the news column filled up with “xxx was stabbed by Cyber-Knight’s knife” (i was a level 4 covert by this time)
those knife kills are great to build up light weapons skill. 
